<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <button>按钮一</button>
    <button>按钮二</button>
    <button>按钮三</button>
</body>
</html>
<script>
    var buttons = document.querySelectorAll('button')
    for(let i=0; i<buttons.length; i++){
        buttons[i].onclick = (function(){
                console.log(i);
        })
    }
    // for(var i=0; i<buttons.length; i++){
    //     buttons[i].onclick = (function(i){
    //         return function(){
    //             console.log(i);
    //         }
    //     })(i)
    // }
</script>